1. Field of the Invention
The present invention relates to a sound converting apparatus for performing conversion between electric signals and ultrasonic waves, and more particularly to a sound converting apparatus operable to perform conversion between electric signals and ultrasonic waves at a low voltage.
2. Description of the Related Art
In recent years, there have been proposed various kinds of sound converting apparatus for performing conversion between electric signals and ultrasonic waves, viz. converting electric signals into ultrasonic waves or converting ultrasonic waves into electric signals used, for example, to probe the internal orgasm of the human body to assist the doctors in diagnosing the human body in the hospitals.
One typical example of the conventional sound converting apparatus is disclosed in Japanese Patent Laid-Open Publication No. 299799/1999. The conventional sound converting apparatus 700 herein disclosed is shown in FIG. 7. The conventional sound converting apparatus 700 is adapted to emit ultrasonic waves converted from electric signals along a wave propagating direction Dp. The conventional sound converting apparatus 700 comprises a plurality of piezoelectric layers 76a, 76b, and 76c, each having a first surface and a second surface, and aligned one after another in a wave propagating direction Dp. The first and second surfaces of the piezoelectric layers 76a, 76b, and 76c are extending substantially parallel to an azimuthal direction Da perpendicular to the wave propagating direction Dp. The conventional sound converting apparatus 700 further comprises a plurality of electrodes, i.e., electrodes 77, 79, 80, and 78 aligned one after another along the wave propagating direction Dp. The electrode 77 is held in contact with the second surface of the piezoelectric layer 76c. The electrode 79 is sandwiched between the piezoelectric layers 76c and 76b and held in contact with the first surface of the piezoelectric layer 76c and the second surface of the piezoelectric layer 76b. The electrode 80 is sandwiched between the piezoelectric layers 76b and 76a and held in contact with the first surface of the piezoelectric layer 76b and the second surface of the piezoelectric layer 76a. The electrode 78 is held in contact with the first surface of the piezoelectric layer 76a. The conventional sound converting apparatus 700 further comprises an electrically conductive film 84 electrically connecting the electrode 77 with the electrode 80, and an electrically conductive film 85 electrically connecting the electrode 78 with the electrode 79. The conventional sound converting apparatus 700 further comprises a signal line 87 electrically connected with the electrode 77 and a signal line 88 electrically connected with the electrode 78. The signal lines 87 and 89 are operative to input an electrical signal to be applied to the piezoelectric layers 76a, 76b, and 76c to operate the conventional sound converting apparatus 700.
As described above, the conventional sound converting apparatus 700 comprising a plurality of electrodes aligned one after another in the wave propagating direction Dp makes it possible to increase the electrical field intensity of an electric signal to be applied to the piezoelectric layers in comparison with a conventional sound converting apparatus comprising a single piezoelectric layer in the wave propagating direction Dp. This means that the electrical field intensity of an electrical signal, i.e., an operating voltage to be applied to the piezoelectric layers of the conventional sound converting apparatus 700 can be less than the operating voltage to be applied to the piezoelectric layer of the conventional sound converting apparatus comprising a single piezoelectric layer in the wave propagating direction Dp. This leads to the fact that the conventional sound converting apparatus 700 is operative at an operating voltage less than the operating voltage which the conventional sound converting apparatus comprising a single piezoelectric layer in the wave propagating direction Dp is operative at.
The conventional sound converting apparatus 700 thus constructed as above described, however, encounters such a problem that the conventional sound converting apparatus 700 is required to comprise electrically conductive films 84 and 85 for electrically connecting the piezoelectric layers 76a, 76b, and 76c. The conventional sound converting apparatus 700 thus constructed encounters another problem that the conventional sound converting apparatus 700 is required to increase the number of piezoelectric layers to be aligned in the wave propagating direction Dp in order to increase the electrical field intensity of an electric signal to be applied to the piezoelectric layers of the conventional sound converting apparatus 700.
The present invention contemplates resolution of such problems.
It is therefore an object of the present invention to provide a sound converting apparatus which is not required to comprise electrically conductive films for electrically connecting the piezoelectric layers.
It is another object of the present invention to provide a sound converting apparatus which can increase the electrical field intensity of an electrical signal to be applied to the piezoelectric layers without increasing the number of piezoelectric layers to be aligned in the wave propagating direction Dp.
It is a further object of the present invention to provide a sound converting apparatus which is simple in construction and operative at an operating voltage less than a conventional sound converting apparatus.
In accordance with a first aspect of the present invention, there is provided a sound converting apparatus for performing conversion between electric signals and ultrasonic waves, comprising: a plurality of oscillation bodies for emitting ultrasonic waves converted from the electric signals along a wave propagating direction; and a plurality of electrically conductive bodies each for electrically connecting the oscillation bodies; a plurality of signal lines for inputting electric signals to be applied to respective oscillation bodies; each of the oscillation bodies including a pair of piezoelectric layers respectively having inner surfaces and outer surfaces, extending substantially parallel to the wave propagating direction, the inner surfaces of respective piezoelectric layers opposing to each other; a pair of external electrodes respectively held in contact with the outer surfaces of respective piezoelectric layers and electrically connected with the electrically conductive bodies; and a dividing electrode sandwiched by and held in contact with the inner surfaces of the piezoelectric layers and electrically connected with the signal line, whereby piezoelectric layers respectively generate electric polarizations, directions of which are opposing to each other and extending substantially parallel to an azimuthal direction perpendicular to the wave propagating direction, and emit ultrasonic waves converted from the electric signals along the wave propagating direction when electrical fields are applied between the external electrodes and the dividing electrode in response to the electric signals. In the aforesaid sound converting apparatus, each of the oscillation bodies has a width with respect to the azimuthal direction and a thickness with respect to the wave propagating direction, and the ratio of the width to the thickness is within a range of from 0.1 to 0.8. In the aforesaid sound converting apparatus, the piezoelectric layers may be disposed in mirror symmetric relationship with respect to the directions of electric polarizations and each of the electrically conductive bodies is operative to electrically connect two oscillation bodies neighboring in the azimuthal direction.
In accordance with a second aspect of the present invention, each of the oscillation bodies may be in the form of a trapezoidal shape in cross section taken on a plane extending substantially parallel to the wave propagating direction and the azimuthal direction. In the aforesaid sound converting apparatus, each of the oscillation bodies has a top surface and a base surface opposing to each other and extending substantially parallel to the azimuthal direction, each of the oscillation bodies has a top width along the top surface and a base width along the base surface with respect to the azimuthal direction, and both of the ratio of the top width to the thickness and the ratio of the base width to the thickness are within a range of from 0.1 to 0.8. In the aforesaid sound converting apparatus, each of the oscillation bodies has a base surface extending substantially parallel to the azimuthal direction, and which further comprises a supporting portion extending substantially parallel to the azimuthal direction, and held in contact with the base surfaces of the oscillation bodies to have the oscillation bodies mounted thereon. In the aforesaid sound converting apparatus, each of the oscillation bodies has a top surface extending substantially parallel to the azimuthal direction and opposite to the base surface, and which further comprises an acoustic matching layer extending substantially parallel to the azimuthal direction, and held in contact with the top surfaces of the oscillation bodies to be mounted on the oscillation bodies.
In accordance with a third aspect of the present invention, the oscillation bodies are one-dimensionally aligned one after another in the azimuthal direction for emitting ultrasonic waves converted from the electric signals along a wave propagating direction perpendicular to the azimuthal direction, and each of the electrically conductive bodies is operative to electrically connect two neighboring oscillation bodies. In the aforesaid sound converting apparatus, each of the oscillation bodies has a length with respect to a longitudinal direction perpendicular to the azimuthal direction and the wave propagating direction, and the oscillation bodies are aligned one after another in the azimuthal direction and in the longitudinal direction. In the aforesaid sound converting apparatus, the ratio of the length to the thickness is within a range of from 0.1 to 0.8. The piezoelectric layers may be made of a material whose transverse electromechanical coupling coefficient (k31) is equal to or more than 35%. Alternatively, the piezoelectric layers may be made of a material of lead zirconate titanate ceramics.